1. Field of the Invention
The present invention relates to a control apparatus for an electromagnetic actuator for driving a mechanical element and, more particularly, relates to a control apparatus for an electromagnetic actuator for driving a mechanical element such as a valve for opening and closing an intake port and an exhaust port of an engine.
2. Description of the Related Art
Conventionally, in some cases, a mechanical element is electrically driven by an electromagnetic actuator in place of mechanically driving it by coupling a cam, a rod or the like thereto. The electromagnetic actuator serves to excite an electromagnetic coil in accordance with an electric signal thereby to drive the mechanical element by the attraction of an electromagnet. The driving timing and the driving force of the electromagnetic actuator can be changed freely by controlling the electric signal, so that the electromagnetic actuator has been employed in many cases in such a field that the precise timing control and variable control are desired.
In a vehicle, the electromagnetic actuators are also employed in various portions in addition to the portion for controlling the opening and closing of an intake valve of an engine. One of the uses to which the electromagnetic actuators are desired to be applied is an intake and exhaust valve for an engine. A system for changing the opening and closing timings of the intake and exhaust valve of the engine depending on the rotation speed range of the engine by using the mechanical structure has been realized. However, if the intake and exhaust valve is driven by the electromagnetic actuator, the opening and closing timings of the intake and exhaust valve can be controlled in various manners and so it becomes possible to improve the output characteristics and the fuel cost of the engine.
In the case of driving the intake and exhaust valve by the electromagnetic actuator, firstly a large overexcitation current for generating energy necessary to start the valve to move from the stationary state is flown into a first electromagnetic coil, and then a relatively small holding current for holding the valve at an opened or closed position is flown into the first electromagnetic coil. Thereafter, a second electromagnetic coil is excited which generates driving force to move the valve to a direction opposite to that moved by the first electromagnetic coil, so that the valve thus moved to the opened or closed position is driven to the closed or opened position by the excitation operation of the second electromagnetic coil in the similar manner as the first electromagnetic coil. When the overexcitation current and the holding current supplied to these electromagnetic coils are cut off, induced electromotive force is generated due to the inductances of the electromagnetic coils, so that the current for continuously attracting the valve at the present position is kept to flow through the electromagnetic coils. As a consequence, since the current flowing through the electromagnetic coils can not be cut off quickly, the responsibility of the valve is degraded.
Japanese Patent Unexamined Publication No. Hei. 8-284626 discloses that a variable resistor unit is inserted into a fly wheel circuit for flowing a current caused by the induced electromotive force which is generated at the time of cutting off a current applied to an electromagnetic coil, and that the resistance value of the variable resistor unit is increased at the time of cutting off the current applied to the electromagnetic coil so that the current caused by the induced electromotive force is rapidly consumed by the resistor thereby to reduce the current.
Further, the publication discloses another embodiment wherein an H-bridge relating to the electromagnetic coil is formed so that, in order to limit the current caused by the induced electromotive force, a current flowing to a direction opposite to that of the current caused by the induced electromotive force is supplied to the coil.
However, according to such a conventional method of reducing the induction current by consuming the electric power through the resistor, the energy generated by the induced electromotive force is wastefully consumed. Further, according to such a conventional method of flowing the current to the direction opposite to that of the current caused by the induced electromotive force, the energy of the power supply is excessively consumed. Furthermore, according to the conventional method, it is required to differ the conduction timing of the current to the opposite direction from the conduction timings of the overexcitation current and the holding current. If the conduction timing of the current to the opposite direction coincides with the conduction timing of the overexcitation current or the holding current, the performance of the switching element of the control circuit may be degraded.